Polyolefin-based adhesives are useful for bonding polyolefins with polar materials such as nylon, ethylene vinyl alcohol copolymer (EVOH), and metals. Polyolefin-based adhesives commonly comprise maleic anhydride-grafted polyolefins. The grafting is usually performed by visbreaking, with or without free radical initiator, the polyolefin in the presence of maleic anhydride to cause formation of free radicals on the polymer chains that react with maleic anhydride. The grafted polyolefin is typically let-down with additional polyolefin resins to a desired concentration and to provide a polyolefin-based adhesive. See U.S. Pat. No. 4,487,885. The grafted polyolefin can be let-down by a variety of polyolefin resins, including high density polyethylene (HDPE), low density polyethylene (LDPE), linear low density polyethylene (LLDPE), polypropylene, and ethylene-vinyl acetate copolymers. High performance adhesives also utilize a polyolefin elastomer as a modifier.
One application of polyolefin-based adhesives is for polymer-metal multilayer pipes. The multilayer pipes are increasingly used for plumbing and heating applications. PE (polyethylene) and PEX (cross-linked PE) are commonly used polyolefins; aluminum and copper are commonly used metals. The multilayer pipes usually have the structure of PEX/tie/metal/tie/PEX or PE/tie/metal/tie/PE. Although the metal provides most of the mechanical strength in these constructions, mechanical integrity and pressure ratings of the multilayer pipes depend on the quality of the tie layer between the metal and PE. There is a need for polyolefin-based adhesives that have improved oxidative stability.